Heat sealing apparatus

ABSTRACT

A heat sealing tray is comprised of a thin sheet of heater element material bonded to an insulating substrate and a rigid backing member. The sheet is etched out in a pattern defining a heating current path. An automatic temperature limiting control circuit is connected to the sheet. The circuit operates in full wave configuration and controls the partial cycle conduction of current through the heating path. A temperature sensor signals the circuit to remove the current through the path when a predetermined temperature is reached.

United States Patent Reenstra et a1.

[451 Oct. 17, 1972 [54] HEAT SEALING APPARATUS 3,060,652 10/1962 Eckman..53/39 X [72] Inventors: John Ream", 1O Ronnie Road 2,825,194 3/1958Page ..93/D1G. 1 3 22:'g zsl h r tgg xt g? Primary Examiner-C. L.Albritton Attorney-Milton Robert Kestenbaum 07506 [22] Filed: April 30,1970 57 ABSTRACT PP' 9 43,627 A heat sealing tray is comprised of a thinsheet of heater element material bonded to an insulating sub- RelatedApphcmon Dam strate and a rigid backing member. The sheet is etched [62]Division of Ser. No. 726,436, May 3, 1968, Pat. out in a patterndefining a heating current path. An

' No. 3,617,696. automatic temperature limiting control circuit isconnected to the sheet. The circuit operates in full wave [52] US. Cl...219/200, 53/373, 219/243 Configuration and controls the partial cycleconduc- [51] Int. Cl. ..H05b 1/00 tion of current through the heatingpath. A tempera- [58] Field of Search ..219/200, 243, 538, 543; turesensor signals the circuit to remove the current 156/515, 583; 93/DIG.1; 53/39, 329, 373 through the path when a predetermined temperature isreached. R f C't d [56] e erences I e 8 Claims, 7 Drawing Figures UNITEDSTATES PATENTS 3,170,275 2/1965 Rohdin et al. 53/373 /a/ /0/ 0, 9 1 Q f0 Y 'l Q 104 I I qg I 95 I 2 l 90 I P99 I 5 l I l l J' o if e i 2? 1 a I:o l 1 J L? r F i 6 4- 1) l 4 -ll 3 912cm ll i 92 I "(Quiz O I 9911/097| -lg C vw l) (U5 4 I k\ 9"q5 J} as IOE 100 105 HEAT SEALING APPARATUSThis is a division of application Ser. No. 726,436, filed May 3, 1968,now U.S. Pat. No. 3,617,696.

This invention relates to heat sealing apparatus and more particularlyto the design and arrangement of heater elements and to means forcontrolling the operating temperature of heater elements.

Heat sealable materials, such as plastic and plasticized materials, whenused in packaging may be sealed serially into pouch-type containers oras separate pre-formed containers which are sealed individually or ingroups. It should be understood that the principles and teachings of thepresent invention are applicable to heater elements used with any formor method of heat sealing, notwithstanding that the embodiment shownrelates to heater elements used for sealing lids onto separatepre-formed containers called blisters.

ln blister packaging, pre-formed plastic containers are placed open sideupwards in individual openings called pockets adapted to receive andhold one or more of them in position for sealing with their edgesextending beyond the opening. The intended contents are placed in eachblister and a lid is placed over each and sealed to a peripheral flangeon the blister through a combination of heat and pressure. One type ofheater element takes the form of Nichrome or stainless steel bands orwires which are arranged in the shape of the planar outline of theblister at its peripheral flange. Blisters of different sizes andconfigurations require different lengths of heater element to affect aseal about them. The power required to bring these heater elements tooperating sealing temperature increases as the total length of heaterelement increases. The problem then is to seal blisters of differentsizes and configurations in the same heat sealing machine. One method isto adjust the power setting of the machine for different blisters. Thisrequires that the machine be adjusted each time a blister requiring adifferent sized heater element is to be sealed and as a practical matterprecludes sealing different blisters at a mechanized rate.

Another method is to have a number of voltage probes serially arrangedin the heat sealing machine in voltage descending steps and a contact.button associated with the heater elements which is pre-positioned tomake contact with the probe supplying the voltage most appropriate forthe power requirements of that heater element. This arrangement isconductive to arcing between the probes and the contact button.

In both of these methods there is a marked tendency for the heaterelements to overheat and burn out the sheet of non-stick material placedover them requiring frequent replacement.

Present heater elements are made up of strips or diecut forms. In theusual case, a number of blisters are sealed at the same time. Each ispositioned in a separate opening on a rigid member known as a heatsealing tray. Each opening is surrounded by its own heater elementhaving a layer of non-stick material thereon. The strips or die-cutforms are connected together by welding or brazing wires to each heaterelement piece and soldering these wires to each other. The number ofopenings per tray is determined by the size of the package to be sealed.

The present method of construction is excessively time consuming andcostly and is inherently unreliable due to the many welded and solderedconnections. The welded wires, which pass through access holes in thetray beneath the heater element are detrimental in two ways: First, theytend to restrain the heater element from expanding when hot and therebycause undue stress which fatigues the element and causes breakage of theheater element or the welded connection; secondly, the wire drains heatfrom the element causing a higher than required operating temperature inorder to effect a seal in the area where the weld is made.

An object of this invention is to provide an apparatus for limiting theoperating temperature of heater elements used in heat sealingapplications.

Accordingly, a further object of this invention is to provide anapparatus for automatically selecting and controlling the powerdelivered to heat sealing heater elements.

Another object of this invention is to provide means for protectingnon-stick material positioned over heater elements from burning out.

Yet another object of this invention is to provide means for sealinglids to pre-formed containers of different sizes, materials,configurations or different combinations of these characteristics,without changing the power setting of the heat sealing machine.

A further object of this invention is to greatly enhance the ease andeconomy of manufacture of heat sealing trays.

An additional object of this invention is to provide heater elements onheat sealing trays without welded or soldered connections to the heaterelements.

Another object of this invention is to eliminate heat drains from heaterelements.

Still another object of this invention is to provide a novel heatsealing tray in which the heater elements are etched from a foil ofheater material.

These objects are accomplished in the present invention by an electroniccircuit arrangement, including silicon controlled rectifiers used infull wave configurations with partial cycle conduction to maintain aconstant root mean square current which is pre-set by resistor selectionto yield the required power for each heater element. For the samepackaging material, the power required is based on the length of theheater element. The temperature of the heater is sensed by a thermistorlocated in close proximity to the heater element. The thermistor outputis applied to a solid state control circuit which when the desiredoperating temperature is achieved removes the control signal from thesilicon controlled rectifiers, removing the flow of current to theheater element.

Advantageously, the temperature control circuit limites the temperatureof heater elements formed by etching the desired heating current path ina foil of heater material. The foil is positioned on a substrate whichis affixed to the rigid tray.

These and other objects and features will be fully understood from thefollowing detailed description taken together with the annexed drawingsin which:

FIG. 1 is a system diagram of the electronic circuit.

FIG. 2 is a detailed circuit diagram of one form of the electroniccircuit employing transistor and relay means for controlling the siliconcontrolled rectifiers.

FIG. 3 is a detailed circuit diagram of a form of the electronic circuitemploying diac and triac means for controlling the silicon controlledrectifiers.

FIG. 4 is a detailed circuit diagram of another form of the electroniccircuit employing diac and triac means for controlling the siliconcontrolled rectifiers.

FIG. 5 is a plan view of an etched heater tray.

FIG. 6, A-F is a partial sectional view through section lines 6-6 inFIG. 5 at successive stages in its fabrication.

FIG. 7 is a sectional view through section lines 7-7 in FIG. 5 showingthe position of the thermistor probe.

The system diagram of the electronic circuit arrangement for controllingthe power and limiting the operating temperature to heat sealing heaterelements is shown in FIG. 1. A silicon controlled rectifier circuit 10is connected between an AC power source 11 and a heater element 12. Acontrol circuit 13 also operates off the power source 11 and isconnected to provide a control signal to the silicon controlledrectifier circuit 10. The control circuit 13 has a power adjust resistor14 therein for controlling the firing angle of the silicon controlledrectifiers so as to control the power to the heater element 12. Athermistor 15 is positioned close to the heater element 12 and isconnected in the control circuit 13 through a control set resistor 16.

CIRCUIT OPERATION When power is applied from the AC power source 11 uponclosure of switch 17, and the heater element 12 is below operatingtemperature, the thermistor 15 is at a high resistance, delivering nosignal to control circuit 13. A control signal is then allowed to passto the silicon controlled rectifier circuit 10 which applies power tothe heater element 12. This power causes the temperature of the heaterelement 12 to rise which causes the temperature of the thermistor 15 torise. When operating temperature is reached, the thermistor 15 becomes alow resistance, and delivers a signal to the control circuit 13 whichabruptly removes the control signal from the silicon controlledrectifier circuit 10. Lack of control signal to the silicon controlledrectifier circuit 10 causes it to block the flow of current to theheating element 12 from the AC power source 11 causing no further risein temperature. Thereafter the temperature of the heater element 12 willdecrease until the resistance of the thermistor drops to a point wherethe control circuit 13 will restore a control signal to the siliconcontrolled rectifier circuit 10. Adjustment of the power is achieved byselection of resistor 14 based on the length of the heater element 12.

Control set resistor 16 is used to adjust the correct operatingtemperature of the heater element 12. Its primary function is to providecompensation for normal variation in the characteristics of theresistance of thermistor 15 and thermodynamic properties of the heaterelement 12 and its environment.

The power and temperature control circuit may be arranged in either oftwo ways:

1. Installation on a moveable tray which contains the opening or othermeans for positioning the blister and the heater element which isadapted to surround the blister. The control circuit 13, siliconcontrolled rectifi- 6 er circuit 10, thermistor l5 and control setresistor 16 may be attached to the tray requiring only the applicationof AC voltage to the tray for operation.

2. Installation in heat sealing machine. The control circuit 13 andsilicon controlled rectifier circuit 10 may be mounted in the heatsealing machine for use with many trays.

Power adjust resistor 14, thermistor 1S and control set resistor 16 aremounted to each tray, which requires six electrical connectors betweenthe tray and machine:

2 for resistor 14 2 for thermistor l5 and resistor 16, and

2 for heater element 12 Tables I and II show the proper resistance ofthe power adjust resistor 14 for a heater element 12 formed ofone-fourth-inch wide by 0.005 inch thick stainless steel band in thetemperature control circuit shown in FIG. 2. Variation of either widthor thickness will require different resistor values as do the circuitsshown in FIGS. 3 and 4. Table I applies to a single heater element or anumber of heater elements connected in series to seal a number ofblisters simultaneously. Table II applies to an even number of heaterele ments for sealing an even number of blisters simultaneously. Theheater elements are divided into two equal sets of serially arrangedheater elements which sets are connected in parallel, e.g. for sixheater elements, within each of two sets of three, heater elements areconnected in series and the sets are connected together in parallel.

TABLE I HEATER ELEMENTS IN SERIES Heater Length-Inches Resistance ofPower Adjust Resistor 25 to 35 10,000 ohms 35 to 45 8,200 45 to 2,700 55to 1,000 65 to 200 TABLE II HEATER ELEMENTS IN SERIES PARALLEL HeaterLength-Inches Resistance of Power Adjust Resistor 50 to 70 10,000 ohms70 to 8,200 90 to I10 2,700 IIO to I30 l,000 I30 to I50 200 The detailsof the electronic circuit for achieving the system functions describedabove will now be described with reference to FIG. 2.

A power source 11 of a fixed voltage of 50 volts RMS 60 cycles persecond has terminals 20, 21 controlled by an on-off switch 17. Aconducting line 22 of 12 gauge wire attached to the tray connects theheater element 12 to these terminals through a pair of siliconcontrolled rectifiers 23, 24 arranged in shunt opposed relationship. Thepower source 11 and its terminals 20, 21 are located in the heat sealingmachine. The heater element 12 is located on a moveable tray whichpasses between a blister loading position and a sealing position beneaththe pressure head of the heat sealing machine. The blister is placed inan opening in the tray to position it open side upwards to receive thecontents to be sealed within the blister. The blister has a planarflange which extends beyond the opening to rest upon the protectivelayer over the heater element.

Each tray may contain an opening for receiving one blister or a numberor openings for several blisters with the heater elements surroundingeach opening arranged in series or in series parallel as abovedescribed.

At each tray opening, means such as depressible pins hold the sealablecards in place over the blister planar flanges.

The blisters, the container contents'and the sealable cards are loadedonto the tray at a loading position which is accessible to the operatorsand away from the pressure head of the heat sealing machine. After thetray is loaded, it is passed beneath the pressure head of the heatsealing machine. When the tray is moved to the heat sealing position,the pressure head becomes actuated to apply pressure to squeeze thecards and the blister flanges against each other. Then in a successivestage of the heat sealing cycle, the heater elements are energized tobring them to heat sealing temperature, after which the thermal energyis removed, the pressure is released and the tray is moved out of theheat sealing position so that the sealed blister containers can beremoved and the trays reloaded.

The terminals 20, 21 are arranged in the sealing machine to make contactthrough suitable connectors in the tray to lines 22 when the sealingcycle commences. This contact may be made by the use of probes in thesealing machine and aligned buttons in the tray which come in contactupon actuation of the pressure head in the heat sealing machine. Thesilicon controlled rectifiers 23, 24 have gates 25, 26 which areconnected through secondary windings 27, 28 of a transformer 29 to thecathode side 30, 31 of their respective silicon controlled rectifiers.

Also electrically connected between the terminals 20, 21 is the pulseforming circuit comprising series connected resistor 32 and power adjustresistor 14, a contact 34 of a relay 33, a capacitor 35, a diac 36 andthe primary winding 37 oftransformer 29.

A third circuit, which has for its purpose to interrupt the flow ofcurrent through the pulse forming circuit, is also electricallyconnected between the terminals 20, 21. This circuit includes the seriesarrangement of rectifier 38, resistor 39, coil 40 of the relay 33 andtransistor 41. Thermistor is connected between the base of transistor 41and to the voltage divider arrangement of resistors 42, 43 through thecontrol set resistor 16. This arrangement is shunted by bias filtercapacitor 44.

Diode 45 is connected across the coil 40 at the collector of transistor41 to suppress the relay arc to protect the transistor.

OPERATION When 50 volts RMS is applied through terminals 20, 21capacitor 35 is charged through resistor 32 and power adjust resistor 14in series with contact 34 of relay 33. For the half cycle when terminalis positive with respect to terminal 21, capacitor 35 is charged suchthat its relay contact side is positive, and the time charge isregulated by the product of its value and the sum of resistor 32 andpower adjust resistor 14. Re-

sistor 32 is a current limiting resistor to protect the diac 36.Capacitor 35 will charge until the voltage across it is sufficient tobreak down the diac 36, which is a device that exhibits a negativeresistance characteristic, i.e. once breakdown voltage is achieved,regardless of polarity, its resistance to flow of current is sharplyreduced. The voltage at which this occurs is between 28 and 32 volts.When breakdown of diac 36 occurs, capacitor 35 discharges abruptlythrough diac 36 causing an impulse of current to flow in the primarywinding 37 of transformer 29. The secondary windings 27,28 oftransformer 29 are connected such that a positive impulse of voltageappears at the gate 25 of silicon controlled rectifier 23. During thishalf cycle, the anode of silicon controlled rectifier 23 is positive anda positive gate voltage will cause it to conduct allowing current toflow through the heater element 12 causing its temperature to rise. Theportion of this half cycle for which current will flow is determined bythe values of resistor 32, power adjust resistor 14 and capacitor 35.Thus silicon controlled rectifier 23 conducts for a pre-selected portionof each half cycle when terminal 20 is positive.

During the alternate half cycles when terminal 21 is positive, capacitor35 charges in the opposite polarity and the circuit operates in a likemanner except that a positive impulse of voltage will be applied to thegate 26 of silicon controlled rectifier 24 causing it to conduct and toallow current to flow through the heater element 12.

Again, the portion of this half cycle for which conduction occurs iscontrolled by the values of resistor 32, power adjust resistor 14 andcapacitor 35 and is the same pre-selected portion as when siliconcontrolled rectifier 23 is conducting.

In both conditions the conduction portion of the cycle is established inaccordance with the length of the heater element 12 and is selected toprovide an RMS or heating value of current which is a constant, i.e. thesame for all heater elements regardless of their length, yielding aconstant power per unit length of heater element. Selection is made byselection of the power adjust resistor 14, for example, as detailed intables 1 and ll for the example of a specific type and size of heaterelement.

The alternating current applied to terminals 20, 21 is also rectified bydiode 38 and the resulting half wave rectified DC voltage is smoothed bythe filter network consisting of resistor 39 and capacitor 44 andapplied to the collector of switching transistor 41 in series with thecoil 40 of relay 33. Resistor 39 is also a current limiting resistor toprotect the diode 38. DC current is also applied through the voltagedivider network of resistors 42, 43 and the series combination of thecontrol set resistor 16 and the thermistor 15 to the base of transistor41. When power is first applied, the thermistor 15 is at roomtemperature and its resistance is very high which allows insufficientcurrent to flow into the base of the transistor 41 to allow it toconduct. The thermistor 15 is in close proximity to the heater elementand its temperature will increase as the temperature of the heaterelement increases. As the thermistor 15 increases in temperature itsresistance decreases allowing current to flow into the base of thetransistor 41. When a temperature is reached where base current issufficient to cause the transistor 41 to conduct, it switches abruptlyto the conduction state causing current to flow through the coil 40 ofrelay 33 causing its contact 34 to open. Opening of contact 34 preventscurrent flow to capacitor 35 which will then no longer supply impulsesto the gates 25, 26 of the silicon controlled rectifiers 23, 24 and nofurther current will flow through the heater element 12 and itstemperature will no longer rise.

Control set resistor 16 in series with the thermistor 15 is used toadjust the temperature at which the transistor 41 will switch. Thisallows for compensation for variations in characteristics betweendifferent transistors 41. If a particular transistor has a low operatingpoint, a larger resistance 16 is selected to permit less current to flowthrough the transistor. Selection of this resistor need only be madeonce upon the initial installation of the temperature control unit inthe tray. Once selected it requires no further adjustment. Its valuewill most often fall within the range of 5,000 to 15,000 ohms.

When the current flow to capacitor 35 is prevented, the heater element12 will begin to cool and the thermistor 15 will increase in resistanceuntil transistor 41 will cut off and relay 33 will drop out, closing itscontact 34. At this point, the circuit will operate to reheat the heaterelement 12. During a normal heat sealing cycle, power is applied to theterminals 20, 21 for only a few seconds after heater shut-off while thetray is under pressure. The cool-down time of the thermistor 15 is about15 seconds and therefore only one heater shut-off operation as abovedescribed occurs during each heat sealing cycle.

Arcing, which has been experienced with probe and contact buttonarrangements in prior art trays, is eliminated by virtue of the actionof the silicon controlled rectiflers and the control circuit. Arcingwill only occur when current is flowing through the circuit.

The silicon controlled rectifiers will block the passage of currentthrough the heater element circuit for the portion of the initial cycleof current during which the impulse forming portion of the controlcircuit is charging up. This may be 10 electrical degrees or moredepending upon the predetermined portion of the half cycle for whichconduction is established. Therefore current will not flow upon closureof the probe and contact button. As above described, current through theheater element circuit is shut off before the heat sealing cycle isconcluded. Hence the silicon controlled rectifiers will again beblocking the flow of current before the probes and contact buttons areseparated.

FIG. 3 shows another embodiment of the temperature control circuit inaccordance with my invention in which a combination of a diac 50 and atriac 58 replace the combination of the transistor 41 and the relay 33of the embodiment shown in FIG. 2, for cutting off the flow of currentto the pulse forming capacitor 61.

The circuit which includes the heater element and silicon controlledrectifiers contains the same arrangement of elements as the circuit inFIG. 2. The pulse forming circuit connected between terminals 20, 21comprises series connected resistor 59 and power adjust resistor 14, acapacitor 61, a diac 60, the primary winding 62 of a transformer 63 anda triac 58.

A third circuit electrically connected between the terminals 20, 21which has for its purpose the interruption of the flow of currentthrough the pulse forming circuit, includes the series arrangement ofresistors 51 and 52, diac 50, and the gate 57 of the triac 58.Thermistor 15 is connected between the terminal 21 and the junction ofresistors 51 and 52 through the control set resistor 16.

OPERATION The operation of the circuit in FIG. 3 is similar to that ofFIG. 2 except that the relay portion has been replaced by a triac solidstate switching device. The impulse forming circuit consisting ofresistor 59, power adjust resistor 14, capacitor 61, diac 60 andtransformer 63 perform exactly the same function as the similar elementsshown in FIG. 1. When power is applied to terminals 20, 21 current flowsthrough resistor 51 and 52, through diac and into the gate 57 of triac58. Triac 58 will conduct causing current to flow through to capacitor61 to form impulses to be applied alternately to the gates 55, 56 ofsilicon controlled rectifiers 53 and 54 respectively. At roomtemperature, thermistor 15 is at a high resistance as the heater element 12 heats up. As the resistance of thermistor 15 decreases, thevoltage at the junction of resistors 16, 51 and 52 will decrease. When atemperature is reached such that the voltage at that junction is lessthan the diac 50 breakdown voltage, current will no longer flow into thegate 57 of the triac 58. The triac 58 will no longer conduct, preventingcurrent from flowing through to capacitor 61 and impulses will no longerbe supplied to the silicon controlled rectifiers 53, 54. I

Again, control set resistor 16 is adjusted to select the desiredtemperature at which the power is shut off to the heater element 12.

FIG. 4 shows a second embodiment of the temperature control circuitaccording to my invention using a combination of a diac and a triac.

The circuit which includes the heater element 12 and the siliconcontrolled rectifiers 77, 78 contains the same arrangement of elementsas the circuits in FIG. 2.

Connected between the power terminals 20, 21 are a triac 73 and theprimary winding 76 of a saturable transformer 75. Thermistor 15 is partof a voltage divider arrangement which includes control set resistor 16and resistor 71 connected in series with it across the power terminals20, 21.

A capacitor 70 is connected through the power ad- 50 just resistor 14 toa junction of the voltage divider arrangement. A diac 72 connects oneside of the capacitor to the gate 74 of the triac 73.

OPERATION With 50 volts RMS applied to terminals 20, 21 circuitoperation is as follows: During each half cycle when terminal 20 ispositive with respect to terminal 21 current will flow through the phaseshift network of resistor 71, power adjust resistor 14 and capacitor 70,causing capacitor 70 to charge such that its terminal 82 is positivewith respect to terminal 81. When the voltage across capacitor 70 issufficient to exceed the breakdown voltage of diac 72 capacitor 70 willdischarge through diac 72 into the gate terminal 74 of triac 73 causingthe triac 73 to conduct and allowing current to flow through the primarywinding 76 of transformer 75. Transformer 75 is a pulse transformer andas such will saturate easily. When saturation occurs, output voltagewill become nil. Since conduction of triac 73 occurs at some point afterthe half cycle has commenced, the wave form has an abrupt rise. Thepulse transformer secondary windings 83, 84 will rise in voltageabruptly at the same time until saturation occurs and then will fallabruptly. The secondary voltage will therefore be a pulse of voltageoccurring at the point in the half cycle at which the triac 73 begins toconduct. The output pulse of secondary winding 84 applied to the gate 79of silicon controlled rectifier 77 will cause it to conduct from thispoint in the half cycle until the end of the half cycle.

During alternate half cycles, capacitor 70 charges in the reversepolarity until breakdown voltage of diac 72 is reached, at which timetriac 73 will conduct in the opposite direction. This will cause reversepolarity pulses to occur at the transformer secondary windings. Theoutput pulses of secondary winding 83 applied to the gate 80 of siliconcontrolled rectifier 78 will cause it to conduct for the partial halfcycle.

When the heater element 12 is cold, thermistor 15 which senses heatertemperature is at a high value of resistance and very little currentwill flow through it. As the heater warms up, the thermistor decreasesin resistance and begins to shunt current from the resistor 14 capacitor70 path. When operating temperature is reached, the resistance of thethermistor 15 is low enough to prevent the capacitor 70 from charging tothe breakdown voltage of the diac 72 and triac 73 is prevented fromconducting, thereby removing pulses from the silicon controlledrectifiers and no increase in heater temperature will occur.

Referring to FIGS. and 6, the etched heater 90 is made from a laminatedconstruction of a metal foil 91 bonded to a high temperature compositioninsulating substrate 92. The binding medium is a flexible temperatureresistant cement. The metal foil 91 is chemically etched through toprovide a continuous electrical circuit with a heater element strip 94surrounding each blister locating opening 93. The strip 94 has asufficiently reduced width to operate at the proper sealing temperaturewhen electric current is applied. The metal areas 95 between blisteropenings are much wider and thereby does not appreciably rise intemperature. The following metals are some of many that might be usedfor the heater foil.

Copper Nichrome Kovar Nickel Nickel-silver lnconel Chromium CopperNickel Carbon Steel Stainless steel lnvar The choice of metals isoptional and is determined by the length and width of the heaterelement. The thickness is also a matter of convenience. However, theoptimum range for etching processes is between 0.004 inch and 0.0l0 inchthick. Some suitable insulating substrates are: Asbestos-filledphenolic; silicon rubber and glass-filled melemine. These may be rigidor flexible and up to one-fourth-inch thick. The bonding cements may beone of a number of known products of laminate manufacturers. These aremostly silicon-base t es.

I 'resent trays have a sheet of non-stick material, such asteflon-coated fiberglass, cemented over the working surface to preventthe plastic blisters from sticking to the heater elements. The thicknessof this sheet and the cement cause a temperature loss which requires ahigher than otherwise necessary heater temperature for a good seal. Theetched heater tray has a sprayed-on and cured in place non-stick coatingof a teflon or silicon-base material, the thickness of which is 0.001inch. This allows a substantially lower operating temperature whichenhances heater life.

In FIG. 5, for ease of illustration, four blister openings 93 ofrectangular shape are shown. Solid lines 96 are etched through to yieldthe heater strip 94 around each blister opening 93. Pins 97 are fixed onthe tray to locate the cards upon the blisters. An area 98 around eachpin 97 is etched through to prevent possible short circuits in case ametal platen is used in the heat sealing machine to apply pressureduring the sealing operation. Broken line 99 shows the path of electricheating current through each heater element and between heater elements.Etched lines 100 are required to block the current flow from unwantedpaths that would by-pass the heater areas. Other etched lines 101 haveno relationship to the heater or electric current flow. However, byinspection it is obvious their presence simplifies the layout process.It is advisable to make a sketch of the layout before proceeding toensure proper flow of current.

One way to construct the heater elements is to first cut out blisteropenings 93 in the laminate in exact locations to those on tray. Thenlines 115 are scribed around each opening 93 the required width of theheater element, usually one-fourth-inch (FIG. 6A) and /s-inch maskingtape 102 laid along the scribe lines 115 (FIG. 68). Lines 100 requiredfor proper electrical flow are added.

Most trays have blister openings93 positioned in lines. Therefore, ascan be seen in FIG. 5, %-inch masking tape strips 102 can be positionedacross the entire tray at the required distance from each opening 93along each side of the blister opening 93. Then by cutting away smallsections of the tape 102 a usable circuit will result. Lines 101 can beleft in place or removed. Then lines 100 that run between openingcenters can be added. The width at 103 is equal to the width of theheating strips around the openings.

In order to make the tray usable in multiprobe type machines, maskingtape is applied in the area 104 where all but the 50 volt probe contactsthe tray. The 50 volt probe will contact the tray along one peripheraledge at 114 and the return probe will contact along the oppositeperipheral edge at 116.

After the masking is complete, a coating of etch resistant material 105is applied (FIG. 6C). When the etch resist 105 is dry all masking tapeis removed (FIG. 6D) and the laminate is etched with a commerciallyavailable etchant such as Hunt Chemical Corporation RCE solutionfollowing the recommendations published by the manufacturer, Etchingtechniques are standard and need not be described here since they areall suitable because of the lack of close tolerance etching requirementsfor this application. After etching is complete, as indicated byvisually noting the complete removal of metal in the desired areas,(FIG. 6E) the etch resist is removed per manufacturers instructions(FIG. 6F). The finished circuit is then rinsed in clear agitated waterto remove traces of etchant and baked at 150 F. for 2 hours to drive offall moisture. After drying, the heater elements are allowed to cool andare sprayed with a non-stick coating such as Dupont Teflon-s, then driedat 100 F. for two hours and baked at 350 F. for one-half-hour to curethe coating.

Referring to FIG. 7, the rigid tray 106 is shown with the laminatecomprising the substrate 92 and the etched heater foil 91 thereon. Ahole 107 is drilled in the substrate 92 behind one of the heater strips94 surrounding the blister openings 93. A corresponding hole 108 isdrilled in the tray 106 so that a probe 111 carrying thermistor may beinserted from below and be in close proximity to the heater element.

The completed heater circuit is attached to the surface of the tray 106by flush machine screws (not shown) or be cementing in place with atemperature resistant adhesive such as Dow Corning Silastic RTV 732.After attachment to the tray, the holes 98 for the card locatedretractable pins 97 are drilled and the pins 97 inserted.

Similar aligned holes 112 are drilled through the tray 106 and thesubstrate 92 for connecting wires 110 of the temperature control circuit109 and the wires inserted from below and soldered to the circuit atconnections 113 in an area outside that which is contacted by the platenof the pressure head in the heat sealing machine.

Alternately, trays intended for use with heat sealing machines thatcontain portions of the temperature control circuit as described abovein connection with FIG. 1, provide for connecting the thermistor probeand the power set resistor to the temperature control portions in themachine. This is accomplished by connecting these components to apredetermined area at the tray periphery to contact a multi-connectionprobe mounted to the platen of the machine.

For etched heaters, it is advantageous that a temperature controlcircuit be used because the cement which holds the metal heater foil 91to substrate 92 will be decomposed at temperatures above approximately450 F., permanently damaging the heater circuit. The non-stick coatingmaterial will also decompose at this temperature. Moreover, contact ofthe power probes is made directly on the circuit (at 114 and 116) andarcing cannot be tolerated since the metal foil is thin. In addition,overheating of the blister material which causes a poor seal, isprecluded by the temperature control circuit.

lt is known that present trays without temperature control which employa teflon sheet over the heater element have overheated to a temperaturesufficient to damage the teflon, which decomposes above 600 F. It can beconcluded therefore, that damaging temperatures will occur if no controlis employed.

In addition to its ease of manufacture and to its elimination of allwelded or soldered connections to the heater elements, the etched heateraffords great versatility in producing heater shapes of any kind withalmost equal case.

Although my invention has been shown and described with reference toparticular embodiments it should be understood that departures may bemade therefrom within the scope of my invention as set forth in thefollowing claims.

What is claimed is:

1. A heat sealing tray having at least one heater element attachedthereto, means for positioning a like number of preformed containersthereon, each of said preformed containers being associated with aseparate one of said heater elements, conductive means for connectingsaid heater elements to AC power terminals, current control meansconnected in the circuit path of said heater elements and adapted foruse in full wave configuration with partial cycle conduction, a controlcircuit adapted to be connected between said power I terminals and tosaid current control means and including impulse forming means foraffecting a predetermined partial cycle conduction of said currentcontrol means and means for impeding the flow of current from said powerterminals to said impulse forming means and temperature sensing meanslocated in proximity to at least one of said heater elements and adaptedto pass a signal to said control circuit when the temperature of saidheater element rises to a predetermined level to impede the flow ofcurrent to said impulse forming means.

2. A tray according to claim 1 in which said impulse forming meansincludes resistance means selected to provide a predetermined partialcycle conduction of said current control means which resistance isselectively decreased as the total linear length of heater element isincreased to provide a constant heating value of current.

3. A heat sealing tray for thermal impulse heat sealing lids topreformed plastic surfaced containers, comprising a rigid backing memberhaving means for positioning at least one preformed container, a pair ofcontacts secured to said rigid backing member for receiving electricalenergy and heater material secured over said rigid backing member andetched through to provide heat sealing elements substantiallysurrounding said container positioning means and the leads forconducting electrical energy to the heat sealing elements, said elementsand conducting leads being integrally formed as an interconnected unitwithout needing additional joining means, in a continuous electricalpath between said pair of contacts,

all portions of said heat sealing elements and all portions of saidconducting leads lying entirely in the same plane over said backingmember and having a constant thickness and uniform composition ofmaterial from one end of said continuous electrical path to the other.

4. A heat sealing tray according to claim 3 wherein all portions of saidpair of contacts lie entirely in said same plane and have a constantthickness and uniform composition of material to said heat sealingelement.

5. A heat sealing tray according to claim 3 wherein said sheet is bondedto an insulating substrate positioned upon said backing member.

6. A heat sealing tray according to claim 3 wherein said heater materialhas a non-stick coating.

7. A heat sealing tray according to claim 3 wherein said preformedcontainer is positioned by aligned openings in said heater material andsaid backing member, said sheet being etched through to provide acontinuous heater element circuit around said opening in said sheet.

8. A heat sealing tray according to claim 3 wherein means are connectedto said heater material for controlling the flow of current in said heatsealing path to limit the temperature thereof.

1. A heat sealing tray having at least one heater element attachedthereto, means for positioning a like number of preformed containersthereon, each of said preformed containers being associated with aseparate one of said heater elements, conductive means for connectingsaid heater elements to AC power terminals, current control meansconnected in the circuit path of said heater elements and adapted foruse in full wave configuration with partial cycle conduction, a controlcircuit adapted to be connected between said power terminals and to saidcurrent control means and including impulse forming means for affectinga predetermined partial cycle conduction of said current control meansand means for impeding the flow of current from said power terminals tosaid impulse forming means and temperature sensing means located inproximity to at least one of said heater elements and adapted to pass asignal to said control circuit when the temperature of said heaterelement rises to a predetermined level to impede the flow of current tosaid impulse forming means.
 2. A tray according to claim 1 in which saidimpulse forming means includes resistance means selected to provide apredetermined partial cycle conduction of said current control meanswhich resistance is selectively decreased as the total linear length ofheater element is increased to provide a constant heating value ofcurrent.
 3. A heat sealing tray for thermal impulse heat sealing lids topreformed plastic surfaced containers, comprising a rigid backing memberhaving means for positioning at least one preformed container, a pair ofcontacts secured to said rigid backing member for receiving electricalenergy and heater material secured over said rigid backing member andetched through to provide heat sealing elements substantiallysurrounding said container positioning means and the leads forconducting electrical energy to the heat sealing elements, said elementsand conducting leads being integrally formed as an interconnected unitwithout needing additional joining means, in a continuous electricalpath between said pair of contacts, all portions of said heat sealingelements and all portions of said conducting leads lying entirely in thesame plane over said backing member and having a constant thickness anduniform composition of material from one end of said continuouselectrical path to the otheR.
 4. A heat sealing tray according to claim3 wherein all portions of said pair of contacts lie entirely in saidsame plane and have a constant thickness and uniform composition ofmaterial to said heat sealing element.
 5. A heat sealing tray accordingto claim 3 wherein said sheet is bonded to an insulating substratepositioned upon said backing member.
 6. A heat sealing tray according toclaim 3 wherein said heater material has a non-stick coating.
 7. A heatsealing tray according to claim 3 wherein said preformed container ispositioned by aligned openings in said heater material and said backingmember, said sheet being etched through to provide a continuous heaterelement circuit around said opening in said sheet.
 8. A heat sealingtray according to claim 3 wherein means are connected to said heatermaterial for controlling the flow of current in said heat sealing pathto limit the temperature thereof.